Stop slots from being executed - c++

Question

In Qt, I can emit a signal, to which I have multiple slots connected, where in the case of direct connections, the connected slots are called on after another.
Let void mySignal(int x) be the signal of the class MyClass.
Depending on the value of x I want to perform a different action, and under the assumption, I want to do exactly one action, I can connect a slot, with a switch-case construct to execute the relevant action.
This implies that I need to know beforehand what kind of values I can get, and what the actions are.
I can also connect a slot for each of my actions, and guard the execution by a if clause. Now I can just connect whatever I want, whenever I want it. But under the assumption that I want to do exactly one action, it would be performance-wise beneficial if I could stop further execution of the slots, when I found the 'match'.
[...]
QObject::connect(this, &MyClass::mySignal, this, [this](int x) {
if (x == 0) {
qDebug() << x; // Stop it now!;
}
});
QObject::connect(this, &MyClass::mySignal, this, [this](int x) {
if (x == 4) {
qDebug() << x; // Stop it now!;
}
});
QObject::connect(this, &MyClass::mySignal, this, [this](int x) {
if (x == 109) {
qDebug() << x; // Stop it now!;
}
});
Is there a way, to tell the signal, to not execute anymore slots, until the signal is emitted again?

Answer 1

One way to do something like this, (ab) using the Qt framework would use the QEvent-system.
The signal handler won't do more, than translating the signal in an QEvent. Instead of all the separate slots, install eventFilter. If the eventFilter accepts the event, return true to stop the propagation to other (filter)objects.
The code is just a quick and dirty test and has no security precautions. It might easily crash hard.
// testevent.h
#ifndef TESTEVENT_H
#define TESTEVENT_H
#include <QObject>
#include <QEvent>
class TestEvent: public QEvent
{
int m_x;
public:
TestEvent(int x = 0);
int x() { return m_x; }
};
class TestEventFilter: public QObject
{
Q_OBJECT
int m_fx;
public:
TestEventFilter(int fx, QObject* parent = nullptr);
bool eventFilter(QObject* obj, QEvent* event) Q_DECL_OVERRIDE;
};
Q_DECLARE_METATYPE(TestEvent)
#endif // TESTEVENT_H
// testevent.cpp
#include "testevent.h"
#include <QDebug>
TestEvent::TestEvent(int x)
: QEvent(QEvent::User)
, m_x(x)
{
}
TestEventFilter::TestEventFilter(int fx, QObject *parent)
: QObject(parent)
, m_fx(fx)
{
}
bool TestEventFilter::eventFilter(QObject *obj, QEvent *event)
{
Q_UNUSED(obj);
TestEvent* e = static_cast<TestEvent*>(event);
qDebug() << "EventFilter for" << m_fx << "got" << e;
if (e->x() == m_fx) {
qDebug() << "accept that event!";
event->accept();
return true;
}
event->ignore();
return false;
}
// run
QObject o;
TestEventFilter* f1 = new TestEventFilter(10);
TestEventFilter* f2 = new TestEventFilter(5);
TestEventFilter* f3 = new TestEventFilter(3);
TestEventFilter* f4 = new TestEventFilter(7);
o.installEventFilter(f1);
o.installEventFilter(f2);
o.installEventFilter(f3);
o.installEventFilter(f4);
qApp->sendEvent(&o, new TestEvent(5));
qApp->sendEvent(&o, new TestEvent(3));
Output:
EventFilter for 7 got 0x369f2fe0
EventFilter for 3 got 0x369f2fe0
EventFilter for 5 got 0x369f2fe0
accept that event!
EventFilter for 7 got 0x369f3250
EventFilter for 3 got 0x369f3250
accept that event!